Secondary emission cathode or dynode of a porous matrix of tungsten or molybdenum impregnated with secondary emission material such as a tungstate or an alkali halide of calcium



Aug. 5, 1969 D. G. KOPPIUS 3,459,986 SECONDARY EMISSION CATHODE ORDYNODE OF A POROUS MATRIX OF TUNGSTEN OR MOLYBDENUM IMPREGNATED WITHSECONDARY EMISSION MATERIAL SUCH AS A TUNGSTATE 0R AN ALKALI HALIDE OFCALCIUM Filed Jan. 27, 1967 STEP 1 COLD COMPACTION OF H TUNGSTENPARTICLES IN DIE INTO BAR SHAPE (GREENBAR) ,-STEP 11 SINTERING OF GREENBAR IN FURNACE TO FORM POROUS MATRIX CHECKING DENSITY OF POROUS MATRIXAND RESINTERING TO PROPER VALUE STEP D1 flSTEP DZ IMPREGNATION OF POROUSMATRIX WITH SECONDARY EMISSION MATERIAL (I.6.,BARIUM TUNGSTATE) STEP IMACHINING OF IMPREGNATED BAR TO DYNODE SPECIFICATION STEPS INFABRICATION OF DYNODE INVENTOR. OTTO G. KOPPIUS ATTORNEY United US. Cl.313346 4 Claims ABSTRACT OF THE DISCLOSURE A secondary emission cathodestructure or dynode formed from a porous matrix of tungsten ormolybdenum impregnated with molten secondary emission material which iseither an oxidic compound, such as tungstates, or a halide compound suchas alkali halides of calcium.

This invention relates generally to secondary emission cathodestructures or dynodes and methods of fabricating such structures.

Almost all metals and some insulators will emit secondary electrons whenbombarded by a stream of primary electrons or ions. The number ofsecondary electrons emitted per primary electron depends on the velocityof the bombarding electrons and upon the nature of the material, as wellas the condition of its surface. The number of secondaries liberated perprimary electron may be greater than one for the velocity of thesecondary electrons is lower than that of the primaries. A secondaryemission element when used as an electrode in an electron tube having acollector electrode or anode for secondary electrons is known as adynode, as distinguished from the term cathode which, when unqualified,is ordinarily an emitter of primary electrons. The terms dynode andsecondary emission cathode are used interchangeably herein.

Currently, the phenomenon of secondary electron emis sion is used in anumber of tubes of commercial significance such as photomultipliers,image converters and storage tubes. Recently the secondary emittereffect has become important in the crossfield microwave tube technologywherein a cold, secondary emission cathode is called for that can standsevere back bombardment under high power requirements. A furtherrequirement usually stipulated is that such a cathode produce no primaryelectrons.

Secondary emission cathodes now in use are generally of the thin filmvariety of the type, SbCo and others. These are completely unstableunder high electron bombardment and at elevated temperatures. It is wellknown, for example, that the alkali halides have a high secondaryemission. However, if a layer of such a compound is bombarded withelectrons, a decrease of the secondary emission yield usually occurs.This happens especially to layers of alkali halogenides evaporated invacuo. The decomposition of the compound, with a consequent decrease insecondary emission yield, is one of the major problems when electronmultiplication is required at high current densities.

Accordingly, it is the main object of this invention to provide asecondary emission cathode structure or dynode which produces no primaryelectrons and which is highly stable under heavy electron bombardment atelevated temperatures.

More specifically, it is the object of this invention to provide adynode constituted by a porous metallic matrix within which a secondaryemission compound is dispersed,

ttes nt the melting point of the compound having been adjusted so thatit will migrate to the surface of the matrix at a given temperaturedepending on the operating characteristics of the specific tube in whichit is installed; this temperature being as low as 600 C. or as high as1100 C., whereby the active secondary emitter material on the cathodesurface is continuously replenished and secondary emission remainsconstant. Briefly stated, these objects are accomplished by:

(A) A porous structure made from either molybdenum or tungsten as thepreferred base material in which the secondary electron emitter isdispersed.

(B) The porous tungsten or molybdenum matrix is impregnated with themolten secondary electron emitter.

(C) The molten secondary electron emitter can be either an oxidiccompound or a halide compound.

(D) The preferred compounds for secondary electron emission at hightemperatures, i.e., 9001100 C., are the tungstates and molybdates ofbarium, strontium, calcium and magnesium. The preferred compound isbarium tungstate. (The tungstates and molybdates may be considered as amixture of two oxides, i.e., barium tungstate BaO-WO barium molybdate-BaO-MoO etc.)

(E) The preferred halides are the alkali halides of calcium fluoride,barium fluoride, magnesium fluoride, and lithium fluoride. These arelisted in order of preference. (Halides are binary compounds of halogen,such as chlorine, fluorine, iodine, etc., with a more electropositiveelement such as sodium, potassium, iron, aluminum, etc.)

(-F) The preferred alkali halide eutectic mixtures are:

Secondary emission cathodes using eutectic mixtures of these fivesystems have been adjusted to operate between about 600 C. to 900 C.with excellent life and high secondary yield.

For a better understanding of the invention, reference is made to thefollowing detailed description to be read in conjunction with theaccompanying drawing whose single figure is a flow chart showing thesteps involved in making the cathode in accordance with the invention.

It has been found experimentally that elastic reflection of the primaryelectrons decreases with increasing energy such that at an energy of 20electron-volts, 40% of the total secondary electrons consist ofelastically reflected electrons, whereas this percentage drops below 10for primary electrons with an energy of electronvolts. (Oneelectron-volt is the energy required by an electron when it isaccelerated through a potential difference of one volt, i.e., 1 ev.)

Within the energy range between 100 and 2000 electron volts the slowelectrons, i.e., true secondary electrons, are the larger fraction ofthe total secondary electron current. It is within this range of energythat most secondary emission effects are measured and made use ofcommercially. Further, a precedent has been established for calling allemitted electrons secondary electrons. It has been tacitly assumed thatall emitted electrons are emitted by the impinging primary electrons.Although incorrect, it is useful when the aim is to find gross effects,i.e., which substances are able to emit very many or very few secondaryelectrons. The yield of secondary electrons is defined as theproportion, 5, of the total emitted secondary electron current to theprimary electron current. The symbol, 6, gives the number of secondaryelectrons emitted by the action of one primary electron. For purposes ofthis patent application only 6 values of greater than one are ofinterest.

The secondary electron emission at very low primary energy, i.e., 10 ev.-or less, has been found to be determined by the work function of thematerial. Since for most practical microwave tubes, the primary energyfar exceeds this value, the work function of the material plays no partin the secondary emission. In fact for most applications a high value ofthe work function is desirable.

The secondary electron emission at high primary electron energy, i.e.,2000 ev. or more, becomes smaller and smaller as the primary electronenergy is increased and one observes more and more inelasticallyscattered electrons. The higher the density of the bombarded material,the greater the number of secondary electrons one observes. Somemicrowave tubes utilize electrons with energies in excess of 2000 ev.and for this reason the porous metal matrix within which the secondaryemitting material is dispersed should be of high density, such astungsten, in order to maximize the number of elastically scatteredelectrons.

The secondary electron emission yield of metals has been found to bealmost completely independent of the temperature. This can be understoodsince the energies of the secondary electrons are large compared to theincreased thermal energy contributed by heating the secondary emittermaterial. With secondary emitter materials involving poor electricalconducting compounds, such as NaF, BaF A1 etc., the situation is morecomplicated. These materials are usually deposited as a layer over ametallic base. Under primary electron bombardment this layer will buildup a charge which would make secondary emission impossible. Since theconductivity of most insulators increases with increasing temperature,then a temperature secondary electron emission dependence is observed.The new cathode which will be described later in greater detail, doesnot show this effect, or at least it is negligible for all practicalpurposes.

The secondary electron emission coefficient, 6, for metals using primaryelectron bombarding energies from 0 to 1500 ev. shows a maximum value ofthe coefiicient. The curve rises rapidly with increasing primary energyto a broad maximum and then slowly declines. The maximum can beexplained by the fact that an increasing primary energy increases thenumber of secondary electrons generated, but the depth at whichsecondary electrons are released also increases and there is anincreasing loss by absorption. For over thirty different metals thecoefficient, 6, has a maximum of 1.8 for planinum and a minimum value of0.5 for lithium. For most metals the coefiicient falls between 1 and1.5. The maximum value of the coefficient, 6, occurs for the majority ofthe metals at a primary electron energy between about 200- 800 ev. Fromthis data it is evident that the choice of the type of matrix base metalwithin which the secondary emissive material is dispersed is notdetermined by the 6 coefficient. Other considerations are more importantsuch as the melting point, thermal stability, chemical activity, etc.,of the matrix metal.

Secondary electron emission from metal compounds is an extremelycomplicated phenomena. Many compounds are not at all stable underelectron bombardment. Most are electrical insulators as a consequence ofwhich surface charge phenomena have a definite influence on thesecondary emission. Technically these substances are even more importantthan the metals since many of the compounds give a high yield. Thehighest values of fi-max. are found with simple compounds of the alkalimetals, such as oxides, halides, etc. Yields are somewhat smaller fromthe simple compounds of the alkaline earth metals, smaller still fromthe oxides of aluminum, silicon, etc. A few examples are given below.

There are a number of complications connected with secondary electronemission from compounds. As was mentioned previously, most areinsulators and the lack of conductivity causes the potential of theemitting surface to be ill defined. In a cathode in accordance with theinvention, the lack of conductivity has been minimized by dispersing theemitting agent in a good conducting metal. The close proximity of themetal matrix to the secondary emission insulator permits the charge onits surface to be removed rapidly.

Further, since the melting point of the emitting agent has been adjustedto be close to the operating temperature of the cathode, theconductivity of the emitter material increases to a value that no chargeremains on the surface.

The most serious limitation of the use of compounds for secondaryelectron emission is their lack of stability under severe electronbombardment. Such compounds initially have a high secondary yield andthen under electron bombardment rapidly deteriorate. For example, anevaporated layer of sodium chloride initially has a 6 coefficient ofabout 5.5 at a primary. electron bombardment energy of 600 ev. whichthen drops to less than 4.0 within about 30 minutes. The acceptedexplanation of this is a bit complicated. The electrons emitted from thesodium chloride layer are those with the highest energy in the crystal,i.e., those occupying the highest energy band. If one of these electronsis emitted a chlorine atom is left in the crystal. Two things couldhappen to the chlorine atom:

( l) the chlorine atom could remain in the crystal lattice, (2) thechlorine atom could escape from the lattice.

Upon the ejection of the electron a positive charge is left behind whichthen must be neutralized by an electron from the base material on whichthe sodium chloride layer is deposited. In the first instance thepositive charge on the chlorine atom is neutralized by recombinationwith the electron from the base. In the second case the chlorine atomhas disappeared from the crystal lattice. Neutrality can be establishedby an electron moving from the base material into the remaining hole.The electron is now trapped and it behaves like a neutral atom. It isknown as a color center. As more and more color centers are formed theremaining sodium atoms form a colloidal dispersion of metal particles inthe sodium chloride layer with the consequence that the secondaryelectron emission drops.

This is because the, 6, coeflicient is less for metallic sodium than forsodium chloride. If the sodium chloride layer is now heated to below itsmelting point the color centers disappear, and the secondary electronemission yield is re-established. The melting point of the secondaryemission material in the cathode according to the invention has beenadjusted such that this phenomena is minimized. That is, the secondaryemission material is dispensed, or rejuvenated in the new cathode.

The requirements for a secondary electron emission cathode for microwavetubes are stringent. The 6 coefiicient should be the order of 2 orbetter. The cathode must be able to withstand considerable back electronbombardment without deterioration. In the event that a primary electronsource is used in the tube to supply the initial electrons, thedeterioration products, if any, of the secondary electron emitter mustnot poison the primary emitter.

Some tubes do not require a primary source of electrons and in thisevent the choice of secondary electron emitter material is much broaderthan in the other case.

Under back electron bombardment the secondary emitter may run as cool as600 C. or as hot as 1100 C. Moreover, in some tubes, due to a suddenchange in load or a change in impedance, a given tube whose secondaryemitting cathode is operating at a temperature of 600 C. may jump to 800C. due to a change in back bombardment. The characteristics of thesecondary emitter must not be permanently afiected by this change intemperature.

A general requirement for most secondary electron emitters is that itmust not emit primary electrons. This requirement becomes more and moredifficult to satisfy as the operating temperature of the secondaryemitter is increased. At the lower temperature, that is, below 700 C.,secondary emissive materials that are good primary emitters at 1050 C.can be used but most of these materials have high melting points and atthe lower temperatures they supply no dispenser action. For this reasonthey may have a good but finite life before the secondary electronemission begins to decay.

For economic reasons, most tube engineers prefer not to put auxiliaryheaters in secondary emitter cathodes in order to properly activate it.Dynodes employing a porous tungsten matrix impregnated with bariumcalcium alumi nate with a mole ratio of 4:1:1 and/ or 5 23:2, require anactivation at 1100 C. in high vacuum for several hours before thesecondary electron emission becomes stable after the cathode temperatureis lowered below 700 C.

Since this cathode is a good primary electron emitter at the activationtemperature, it must be operated as a secondary emission cathode below700 C. if primary electrons are to be avoided. At this temperature nodispenser action of the emitter material occurs and the secondaryemission has a limited life. Since most microwave tubes are processed ata bake-out temperature of about 600 C., a secondary emission cathodethat will activate at this temperature is desirable since in this eventno auxiliary heaters are necessary. Finally, the secondary electroncathode must be stable physically, that is, its geometricalconfiguration must not change, also there is a preference for cathodesmade from non-magnetic materials.

Porous tungsten and molybdenum have been found to be the most desirablematrix base metal. They are not magnetic and are stable geometricallyunder the conditions of tube operation. The preferred method of makingporous tungsten and/or porous molybdenum involves Steps I through V, asdescribed below for tungsten which includes the impregnation of theporous tungsten metal with one of the preferred secondary emissivecompounds, barium tungstate.

Step I.Tungsten powders having a mean particle size of 4.5 microns arepressed in a suitable die into bars or other suitable shapes, using apressure of 10,000 to 30,000 psi, preferably of 20,000 p.s.i. Such greenbars, even without the use of a binder, have sufiicient coherence andstrength to be handled, and they can be readily shaped by hand tools.

Step II.The green tungsten bars are then loaded into a high-temperaturesintering furnace where the temperature is slowly raised to about 2360C., and maintained at this level for about 20 minutes. It has been foundthat for 4.5 micron commercial tungsten powders pressed at 20,000 psi.and sintered at 2360 C. for 20 minutes, the resultant structure has adensity of 84% of theoretical. 'It will be appreciated that by properadjustment of time and sintering temperature parameters, other values oftheoretical density may be obtained.

Step IIL-For purposes of quality control, the porous tungsten barsformed in Step II are weighed first in air and then weighed again whensubmerged in mercury. From these two values the percentage oftheoretical density can be computed. If the percentage falls below thedesired amount, the bars can be resintered until the requisite value isattained. To insure uniform skeletons,

the theoretical density should be held to within plus and minus 2%.

Step IV.The porous sintered tungsten bars are then loaded into ahigh-temperature impregnation furnace having a hydrogen atmosphere. Anamount of barium tungstate sutficient to effect complete impregnation isplaced around and in contact with the bars. The furnace temperature isthen slowly elevated until the barium tungstate becomes molten andstarts to wet the porous tungsten skeletons. At this point, thetemperature is 1650 C., the melting point of barium tungstate, which isstill well below the sintering temperature of the tungsten. Thistemperature level is held for about 10 minutes, and as a result, allpores of the tungsten skeleton are filled with the molten bariumtungstate. The furnace temperature is then quickly reduced to roomtemperature.

Step V.-Finally, the tungsten bars impregnated with barium tungstate aremachined to the required dynode specifications to produce a secondaryemission structure.

In addition to barium tungstate, the following compounds are goodsecondary emitters having a 5 coefficient of about 3. These are classedas high temperature melting compounds with a very poor to poor primaryemission at 1050 C.

The above compounds are exceedingly stable under electron bombardment,and work well as secondary electron emissive material when impregnatedinto either porous tungsten or molybdenum. Compounds like barium calciumaluminate have to be activated at 1100 C. in vacuum for a period of timein order to have a stable secondary electron emission yield attemperatures below 700 C.

The most important compounds for secondary electron emission yield athigh temperatures are the tungstates and the molybdates of barium,strontium, calcium and magnesium. Of these barium tungstate is thecompound preferred. It is the most stable one chemically. All of thetungstates and molybdates give no primary emission at 1050 C. providedof course the compounds are of high purity. The secondary electronemission coefiicient, 8, has a value between 2 and 3.

Porous tungsten dynodes having a theoretical density between 84% of puretungsten impregnated with molten barium tungstate have been found tohave thousands of hours life when run between 900 C. and 1050 C. inmagnetrons under severe bombardment. The operation temperature, i.e.,900-1050 C. is close enough to the melting point of barium tungstatethat rapid diffusion of the emitter material occurs over the tungstensurface and a dispenser action maintains the emitter material on thesurface. Further, the operating temperature is suificiently high thatthe color centers are destroyed as fast as they are formed and thesecondary electron emission yield holds at a high level. If thesecondary emitter temperature is decreased to below about 700 C. thensuch a cathode has only a life of a few hundred hours beforethesecondary emission drops. No dispenser action occurs at thistemperature.

The highest yield of secondary electrons has been obtained from thefluorides of calcium, barium, sodium, lithium, and magnesium. These,when mixed together, form low temperature melting eutectics. Further,they have been found to wet in the molten state both porous tungsten andporous molybdenum. The impregnation of the porous metal matrix by thesemolten alkali halides is complete. Also and most important, theresultant impregnated metal matrix can be machined with ease byconventional machine tools. In this case the fluoride impregnant actsvery much the same for machining purposes as if the porous matrix waseither impregnated with molten copper or a plastic.

The preferred secondary electron emitter is calcium fluoride as it isthe most stable and it has been found to have a high yield of secondaryelectrons. The coefiicient, 8, exceeds 3 and in a number of tests was ashigh as 8. The order of preference after calcium fluoride is bariumfluoride, magnesium fluoride, sodium fluoride, and lithium fluoride.The, 6, coefficient for these is about 5. The melting points of thesealkali halides are listed below:

c. can: 1418 Bar, 1280 M r 1270 NaF 980 These secondary electronemitters can be used individually, as a combination of two compounds, asa combination of three compounds, and even as a combination of fourcompounds.

The following five fluoride systems of three compounds each have beenfound to have extraordinarily good secondary electron emissionproperties in that the yield is above 5 for all combinations:

The melting point of the eutectic mixtures can be changed from a lowvalue of about 600 C. to about 1200 C. by changing the amount of each ofthe three compounds used to form the emitter material.

Since the melting points of the fluoride systems can be changed over awide range it is possible to pick an optimum mixture which has a meltingpoint sufficiently close to the operating temperature of the secondaryemitter that dispenser action occurs. Further, under these conditionsthe color centers formed during bombardment by the primary electrons aredestroyed and the yield of secondary electrons continues at a highlevel. Finally, since the melting temperature of the alkali halidemixtures can be adjusted to a value close to the bake-out temperature ofthe tube, no further activation of the secondary emitter cathode isrequired.

The fluoride systems are most useful in those microwave tubes requiringno source of primary electrons. In tubes using a primary electron sourceit is necessary to use a very good barium getter to insure continuedprimary cathode activity.

While there have been shown preferred embodiments of the invention, itis to be understood that many modifications may be made therein withoutdeparting from the essential spirit of the invention as defined in theannexed claims.

What I claim is: 1. An electron tube having a dynode and means tosubject said dynode to electron bombardment to an extent causing it tooperate at a temperature in a range of 600 C. to 1100 C. withoutemission of primary electrons, said dynode comprising:

(A) a matrix formed of a porous metal selected from a class consistingof molybdenum and tungsten, and

(B) a secondary emission material impregnating the pores of said matrixand having a melting point at which, in the operating range of thedynode, causes the material to migrate to the surface of the matrix,said material being selected from a class consisting of an oxidiccompound and a halide compound.

2. A dynode, as set forth in claim 1, wherein said oxidic compounds arethe tungstates and molybdates of barium, strontium, calcium andmagnesium.

3. A dynode as set forth in claim 1, wherein said oxidic compound isbarium tungstate.

4. A dynode as set forth in claim 1, wherein said halide compounds arethe alkali halides of calcium fluoride, barium fluoride, magnesiumfluoride, and lithium fluoride.

References Cited UNITED STATES PATENTS 3,297,901 1/1967 MacDonald et al.313-346 3,118,080 1/1964 Koppius 313-337 3,197,662 7/1965 Schneeberger313---1O4 JOHN W. HUCKERT, Primary Examiner B. ESTRIN, AssistantExaminer US. Cl. X.R. 252-521; 313-337

